You can extend it. March of 1981 Hinckley shoots Ted Kennedy instead of Reagan, but Kennedy doesn’t survive. That would make Kennedy ‘s VP the 9th President in less than 18 years.
To say nothing of Mark David Chapman who could easily choose to go after a president, Sarah Jane Moore if your Ford POD is Fromme’s bullet, and the international presence of attempts against Elizabeth II, Pope John Paul II, and Anwar el Sadat.
Maybe I’ll rewrite Passkey Down at one point…